The present invention relates to an improvement to a lubrication circuit of a vacuum pump, for the purpose of achieving numerous functions which the pump can perform by using the lubrication circuit; this is all achieved without constructional complications and with minimum additional cost.
In order to be able to reach high vacuum values, rotary pumps generally use oil as a dynamic seal for sealing coupling clearances. In order to enable the oil also to exert an indispensable lubrication action between parts moving relative to one another and to dissipate heat, it is necessary to ensure adequate circulation of the oil inside the pump.
In pumps at present on the market this circulation is achieved principally in two ways:
(a) through reduced pressure: the oil is drawn back into the vacuum pump by way of calibrated ducts through the difference in pressure between the outside and the inside;
(b) through forced circulation: the oil is driven into the interior of the vacuum pump by means of an auxiliary pump.
It is obvious that in the first solution, which is constructionally simpler (not requiring means for energizing the oil), it is not possible to take advantage in any way of the lubrication circuit to enable it to perform functions other than lubrication. Another disadvantage of this solution is that it does not permit prolonged operation of the pump at relatively high suction pressures. Under such conditions, in fact, the difference in pressure between the outside and the inside of the pump is not sufficient to enable the oil to overcome the resistance that it encounters when it attempts to enter the pump; in this situation there is consequently a risk of seizure.
The presence of an auxiliary lubrication pump not only enables the abovementioned disadvantages to be completely overcome, but also makes it possible to use the oil pressure for controlling devices whose presence is indispensable for correct and reliable installation of the pump in plant which must be exhausted. The most important of these device is the nonreturn device. In the event of the failure of the pump or of a sudden interruption of the supply, there is in fact a risk that first the oil contained in the casing and then the air will be drawn back into the pump by the reduced pressure prevailing there, and will then return into the exhausted installation by way of the suction duct, with serious consequences for the quality of the work being done and the contamination of the installation itself. It is therefore indispensable that the pump should be equipped with an appropriate nonreturn device completely isolating the suction duct from the atmosphere.
Various devices exist which enable this aim to be achieved; these may be divided into two main categories:
(a) devices maintaining the entire pump under vacuum;
(b) devices which maintain the pump at atmospheric pressure.
Those of the first category are the simplest in construction and consist of devices preventing the admission of oil or air into the pump when the latter is at a standstill. These devices can basically be constructed in two different ways, both of which are intended to close the oil admission holes of the pump when the latter stops. The first solution provides for the use of a centrifugal device and is normally adopted in pumps lubricated by suction. The second consists of a calibrated relief valve and is normally used in forced lubrication pumps; when the pump stops, the pressure drops and consequently the relief valve closes the oil supply duct of the pump.
In order to ensure the maintenance of the vacuum inside the pump, both these systems require the use of gaskets - generally of elastomer material - to form seals between the various component parts of the pump; since some of the gaskets intended to ensure dynamic leaktightness during the operation of the pump must ensure perfect static leaktightness (not normally their purpose) in order to keep the pumps under vacuum, there is actually an increased probability that leaks will occur and that tightness cannot be ensured. Finally, the centrifugal device referred to, when it is used, is normally in the form of resilient members in continuous movement and therefore subject to deterioration due to wear and/or fatigue.
It is in addition necessary to take into account two phenomena which limit the efficiency of nonreturn devices which leave the pump under vacuum:
(1) The pump under vacuum remains in communication with the suction line and, if other valves are not provided, also with the installation; since the temperature of the pump is normally higher than that of the suction line and of the installation, the oil vapors contained inside the pump tend to condense on surfaces outside the latter, and consequently also in the suction duct;
(2) When the pump is stopped with the ballast valve (for the elimination of condensable vapors) open, it is not possible to avoid the undesirable return of oil and ballast gas along the suction duct and thus to prevent the pressure in the installation from rising again.
For these reasons the nonreturn devices which leave the pump ensure greater reliability of the system. The gaskets between the various parts of the pump are in fact not required, since the same pressure exists both outside and inside the pump. Furthermore, the device is operated only on the stopping of the pump, thus drastically reducing the number of possible breakdowns due to wear or fatigue. A device of this kind is generally composed of a small piston slidable in a cylinder and received in a closure member floating on it. When the pump is stopped, a valve whose open and closed positions are brought about by the operation of the pump enables fluid at a higher pressure than that inside the pump to enter the cylinder. The piston thus slides in the cylinder and the closure member forms a seal against a seat, which is generally formed near the suction duct. In simpler cases the fluid used is atmospheric air or the air present inside the pump casing.
A centrifugal device connected to the rotor of the pump or a solenoid valve connected to the supply system of the pump, or fed by a generator fastened to the pump shaft, brings the cylinder into communication with the air when the pump is stopped. The pressure of the air then moves the piston and brings the closure member against the seal seat of the suction duct.
With a system of this kind, however, a part of the air will penetrate into the pump during the stroke of the piston, passing through the clearance between the piston and the cylinder, and will have time to pass also into the suction duct before the closure member comes to lie sealingly against its seat. This causes the pressure to rise again in the pump suction line, which is undesirable.
In order to prevent this from happening, systems have been evolved which, because of the pressure produced by an auxiliary oil pump and with the aid of an appropriate circuit, make use of the oil to operate the nonreturn device, thus preventing the air from entering the pump before the suction duct has been completely closed by the closure member.
One example of a system, of this kind makes use of the flow of pressurized oil produced by the oil pump. The oil pressure on the delivery side is kept constant by a breather valve. A second duct, branched off from the supply duct, allows a certain oil flow to pass. Because of its pressure, this oil flow pushes a piston, against the action of a spring constituting the control device, to close the aperture bringing the circuit into communication with the cylinder containing the piston of the nonreturn device. Through the action of the pressure, the oil flow passes on the sides of the piston and, after flowing above it, passes out via a hole formed in its housing and fills an uncovered chamber, from which it overflows to return to the casing. When the pump is stopped, the oil pressure falls abruptly and the spring pushes the piston and brings the cylinder of the nonreturn device into communication with the chamber previously filled with oil. The pressure prevailing in the casing causes the oil in the chamber to pass through the aperture and move the piston of the closure member. In this phase, during the stroke of the piston, it is the oil itself that operates the piston to seal the clearance between it and the cylinder and prevents air from entering. When the oil contained in the chamber has been discharged, the closure member will already have reached its sealing position against the seat of the suction duct and, since there is no longer any oil there to make a seal, the air can enter the pump by travelling along the same path as that previously travelled over by the oil.
This system successfully achieves the aim of controlling the nonreturn device in dependence on the operating conditions of the pump, of causing the control device to act only when the pump is stopped, and preventing oil and air from passing upwards again in the suction duct. However, this solution has some disadvantages:
(a) Constructional disadvantages:
it is necessary to form a suitable seat to receive the piston of the control device and it is also necessary to provide the respective ducts, with the consequent increase in size and additional work;
the auxiliary oil pump must be sufficiently large to provide a far greater flow than that required for the vacuum pump. The piston of the control device is in fact fed in parallel with the pump and is operated by the pressure drop of the oil flow through the clearance between the piston and the seat. This drop is dependent on the clearance existing, which in order not to have an excessive influence on costs must be fairly large. This entails the need for consistent flows in order to achieve the opportune drop. In view of the great variability of operating temperatures and hence of viscosity, such flows make it necessary in practice to adopt oil pumps of the positive displacement type, with their resulting cost and constructional complications;
(b) Functional disadvantages:
the flow of oil, the pressure drop of which operates the control device by collecting in a chamber, is also used as fluid for operating the piston of the nonreturn device. This oil comes from the casing and during its movement is subject to turbulence, so that to a certain extent an emulsion is formed with air. Since however there is a continuous flow, the oil collecting in the chamber does not have time to free the air mixed with it. When the pump is stopped, the oil operating the nonreturn device therefore brings into the interior of the pump a certain amount of air, which causes the pressure to rise again in the exhausted system;
the superabundant flow provided by the oil circulation pump gives rise to the undesirable generation of heat; this necessitates the use of additional heat disipation means in order to ensure the optimum operating temperature of the pump;
since the control device of the nonreturn valve is operated by an oil flow in parallel with that circulating in the vacuum pump lubrication circuit, it is only when the operating pressure has been restored in the lubrication circuit that the control device is moved from the position of rest and the nonreturn valve is opened; this means that it is necessary to wait a not inconsiderable time before the vacuum pump, when it has been put back into operation after a stop, can resume pumping from the suction line.